Influenza surveillance in Europe: establishing epidemic thresholds by the moving epidemic method

Abstract

Background: Timely influenza surveillance is important to monitor influenza epidemics.

Objectives: (i) To calculate the epidemic threshold for influenza-like illness (ILI) and acute respiratory infections (ARI) in 19 countries, as well as the thresholds for different levels of intensity. (ii) To evaluate the performance of these thresholds.

Methods: The moving epidemic method (MEM) has been developed to determine the baseline influenza activity and an epidemic threshold. False alerts, detection lags and timeliness of the detection of epidemics were calculated. The performance was evaluated using a cross-validation procedure.

Results: The overall sensitivity of the MEM threshold was 71·8% and the specificity was 95·5%. The median of the timeliness was 1 week (range: 0-4·5).

Conclusions: The method produced a robust and specific signal to detect influenza epidemics. The good balance between the sensitivity and specificity of the epidemic threshold to detect seasonal epidemics and avoid false alerts has advantages for public health purposes. This method may serve as standard to define the start of the annual influenza epidemic in countries in Europe.

Figure 1

Epidemic threshold, levels of intensity and modelled influenza‐like illness season 2009–2010 by country.

Figure 2

Epidemic threshold, levels of intensity and modelled acute respiratory infection season 2009–2010 by country.

Figure 3

Time series of influenza‐like illness by country, the epidemic periods modelled by MEM, and the alert week according with the pre‐epidemic threshold of each season.

Figure 4

Time series of acute respiratory infection by country and the epidemic periods modelled by MEM, and the alert week according with the pre‐epidemic threshold of each season.